The '259 document discloses that an industrial textile can be assembled from at least two layers of selectively slit and embossed polymeric film. As disclosed, the initially planar film is slit according to a chosen slitting pattern to provide permeability, and then deformed in a controlled process to form protrusions extending above the plane of the film, and land areas in the plane; two layers of film are joined by means of a chosen bonding process. The order in which these processes of bonding, slitting and deformation are performed can be selected depending on the intended end use properties of the film. The slitting and deformation processes form apertures beneath the protrusions at locations in the film where it is embossed. The '259 document discloses that industrial textiles having utility in industrial conveying and filtration processes such as papermaking and the like can thus be formed; other uses are possible and are within the scope of the invention.
In the papermaking process, a dilute slurry consisting of less than 1% papermaking fibers together with a mixture of about 99% water and other papermaking components is ejected at high speed and precision from the slice opening of a headbox onto a moving forming fabric. The fabric is guided and driven by a number of rolls over various drainage boxes and foils which assist in the removal of water so as to leave behind a randomly dispersed, loosely cohesive network or web of papermaking fibers. At the end of the forming section, this web is transferred to the press section, where further water removal occurs by mechanical pressures as the web is conveyed on or between a series of press fabrics through one or more nips. The now self-supporting but still very wet web is then transferred to the dryer section of the papermaking machine where it is conveyed in serpentine fashion around the periphery of many steam heated dryer cylinders (or cans) while supported by several dryer fabrics until the remaining water is removed by evaporation. The resulting paper product may then be exposed to various treatments before it is then finally wound onto a reel, cut to size and packaged for shipment.
As a dryer fabric conveys the sheet through the dryer section, it passes at speed around the periphery of numerous rolls, including the heated dryer cylinders themselves as well as various smaller diameter guide rolls, vacuum rolls, return rolls, and so on. In so doing, an outer surface of a first film layer of these double layer fabrics will travel a greater distance than an inner surface of a second similar film layer (see, for example, EP 806519, para. 0009, line 20). The difference between the travel distances of each of the two layers becomes more pronounced as roll diameters decrease. This can cause internal stresses in the fabric which may eventually lead to premature delamination of the two film layers.
In between the exterior surfaces of the two fabric layers there also exists a planar region that undergoes zero strain as the fabric bends and wraps about these rolls; this region exists in all textiles and is referred to as the neutral plane. The neutral plane always travels at the same speed regardless of the radius of curvature to which the fabric is exposed.
In instances where the film fabric is created by aligning and joining the two layers with their flat planar surfaces together, such as shown in FIG. 11a or 11b of the '259 document, so that the contact and joining region of the two layers will be at or near the neutral plane in the interior of the fabric, the protrusions on the two outer surfaces will be subjected to minimal, if any, mechanical stresses. However, when the fabric is assembled such that the protrusions of each layer are mated to the land areas of the other, the neutral plane will not coincide with either of the surfaces where the joints are formed. When in use in an industrial environment where the fabric must repeatedly pass about the circumference of relatively small diameter rolls, the repetitive stress and strain on these bonds due to differences in travel distances between each of the two layers will cause the joins (in particular welded joins) to flex repeatedly, possibly leading to their premature failure due to in-line shear forces.
If, however, the joints between the two film layers are sufficiently flexible, such that one film layer can slide and move to a small extent relative to the other, then any stresses created by the deformation of the fabric and between the two layers as they curve about one or more rolls can be significantly reduced or eliminated, to avoid delamination of the fabric. The present invention provides a means whereby such relative movement of the two film layers in these fabrics can occur as they wrap portions of the peripheries of small diameter rolls, without adversely affecting bond strength between the two film layers.
According to the invention, at least some of the protrusions formed in a first suitably embossed film layer intended for use in a nonwoven industrial fabric such as that described in the '259 document are shaped such that a first end of the protrusion is integral to and contiguous with a land area in the film, and an opposing second end is either completely detached from the film, or is weakened such that, under strain, it is detachable from the film, so as to form a “tab” or tab-like configuration. When the first film layer is joined to a second mutually compatible film layer during assembly, the first layer is bonded to the second only to the upper surfaces of the detached or detachable protrusions. Because the tab-like protrusions are either detached or are detachable from the film at one end only, movement of the first film layer relative to the second is now possible while still maintaining a strong bond between the two film layers. The remaining protrusions, in which both the forward and rearward end walls are contiguous and integral to the film layer in which they are formed, and may have any suitable configuration, for example one which is compression resistant or otherwise intended to enhance selected fabric properties. In this way, the resulting nonwoven film fabric retains properties and characteristics similar to those of a woven textile. The number and location of non-tab-like protrusions on the first film layer are selectable in accordance with desired end use characteristics of the film fabric.